Automatic injectors have been developed specially for use by persons who for some reason must inject a solution into their own body at a time which cannot be predicted, e.g. soldiers who have been subjected to nerve gas in war. These injectors are stored for years and are further more often subjected to hard conditions during the time the potential user carries it, which sometimes may be long. When the automatic injector finally is to be used it must work with a great reliability when an injection is to be carried out. At such a critical moment it is important that the injector can be handled and activated in a simple and quick manner. Consequently, there are special requirements on automatic injectors of the type mentioned above regarding reliability and simplicity in both handling and operation.
Lately effective antidotes against nerve gases have been developed, said antidotes being stable in the form of a powder, but having proved difficult to make stable in solution, especially during the long storage periods being normal in this field of application. Consequently there is a demand for automatic injectors which are simple, quick and reliable and in which it is possible to store two substances separately for a long period and wherein said substances also easily can be mixed before use.
Some different types of automatic two chamber injectors are previously known:
U.S. Pat. No. 4,529,403 discloses an automatic two-chamber injector with an ampoule located between the injection needle and the plunger. The ampoule is provided with one or several pistons of flexible material, as rubber, keeping the inJection solutions separated from each other. In the front part of the ampoule there are sidewardly arranged channels or sections, which permit the injection solutions to pass the piston or pistons.
U.S. Pat. No. 4,413,991 discloses another system for an automatic two chamber injector, in which a plunger drives an injection needle through two separate chambers containing different solutions. The injection needle is provided with side-openings interconnecting the two chambers during injection.
U.S. Pat. No. 4,202,314 discloses a system with two chambers whose contents are mixed and then automatically injected. Two driving systems are arranged, one for mixing and the other for injection. The injection needle is located inside the cartridge.
In the embodiment disclosed in U.S. Pat. No. 4,214,584, separate chambers are concentrically arranged with and separated by a cylindrical wall from a first chamber. In order to mix the contents of the chambers, the cylindrical wall is screwed upwards by which means the chambers are united with the first chamber. In this embodiment as well, the injection needle is arranged inside the cartridge prior to use.
The previously known automatic two chamber injectors have a number of disadvantages:
The use of polymer material to separate a solution from a powder, alternatively a solution from a solution, has the consequence that a permeation of liquid to some extent is inevitable during a long storage period.
The mixing of the separated medical substances is incomplete and may in some cases be non-existent.
To mount the injection needle inside the cartridge in an injection solution involves a great risk that the injection needle will be corroded during a long storage period.
If a complicated operation to achieve a sufficient mixture of the substances has to be carried out, in conjunction with a lack of a precise control of the operation, there is a risk that the injector may be used in a erroneous way, causing non-appearance of medical effects and even risks for accidents.
There is no possibility to replace the cartridge whilst preserving the sterile conditions.